The present invention relates to a single ion implantation system which utilizes a focused ion beam (FIB) or micro ion beam (MIB) by an ion microprobe and, more particularly, to a single ion implantation system which permits highly accurate implantation of a single ion or predetermined number of controlled ions into a target area or region with a predetermined high degree of aiming accuracy.
In ultrafine-microstructured devices such as a next-generation insulated gate field effect transistor (MOS FET) of a size below 0.1 .mu.m and a quantizing functional device which is a direct extension of microminiaturization technology, the impurity concentration, carrier density and surface and interface levels decrease to such an extent that they could counted; hence, their fluctuations would exert enormous influence on electrical characteristics of the above-mentioned ultrafine-microstructured devices. For this reason, the implementation of the ultrafine-microstructure calls for extremely high precision controllability of fabrication techniques, but many potential factors which would seriously impair the required controllability, such as damage during fabrication and the influence of hidden impurities, lurk in the fabrication process. Since such ultrafine-microstructured devices could not be implemented unless these factors are cleared away one by one, it is a significant challenge at present to seek fundamental or full elucidation of an elementary process for the manufacture of the ultrafine microstructure and its solid-state character control.
A description will be given of the prior art for solid-state character control of the ultrafine microstructure.
(1) Interface Control
A metal/semiconductor interface is one of main themes in solid-state physics that has been studied for the past tens of years, yet its structure and property have not completely been clarified: for example, even in the case of the Schottky barrier forming mechanism, a number of models have been proposed and no theory has been established so far. Above all, as regards the ohmic contact through which a signal is taken out from a device, even an empirical formula for device simulation has not been obtained yet. The reason for this is that the interface is a region buried in the bulk so that direct measurements of its characteristics are extremely difficult. It is another reason that conventional analysis means for the evaluation of the interface provides only averaged information on nonuniform or inhomogeneous interface resulting from chemical reactions, formation of defects and interdiffusion that are associated with the formation of the interface. In other words, characteristics of the metal/semiconductor interface could not completely be understood without analysis means which permit direct evaluation of the interface at high spatial resolution.
A ballistic-electron-emission microscope (hereinafter abbreviated as BEEM) developed by Kaiser and Bell of JPL in 1988 is an improvement on a scanning tunneling microscope (hereinafter abbreviated as STM) for application to the evaluation of the interface [W. J. Kaiser and L. D. Bell, Phys. Rev. Lett., 60, 1406 (1988)]. The BEEM utilizes the fact that in a metal thin film/semiconductor interface system, those of electrons constituting a tunnel current between the chip and metal of the STM which has reached the metal surface partly conduct through the metal film in a ballistic mode to the metal/semiconductor interface. The BEEM allows measurements of the Schottky barrier height (SBH) in a region within 2 nm through utilization of its voltage-current characteristic. Hence, the BEEM could serve as potent evaluation means for clarifying physical phenomena in the metal/semiconductor interface, but substantially no study is being given the BEEM in Japan. The inventor of this application has made the BEEM for the first time in Japan by modifying the hardware of the STM and started measurements of the Schottky barrier height (SBH) in a Pt/n-Si (100) interface by the BEEM and studies on a microscopic spatial distribution of the Schottky barrier height. With the single ion implantation system according to the present invention, the atomic elementary process of the interface reaction can be made clear by observing the effect of manipulation on the interface by single ion implantation through maximum utilization of the high spatial resolution of the BEEM.
An SiO.sub.2 /Si interface system has been investigated very actively as the most important constituent of the MOS FET. Recently, with a view toward the formation of an extremely thin gate oxide film for the next generation of MOS FETs, many studies are being made on the silicon surface condition prior to oxidation, the process of formation of natural oxide films [M. Morita, T. Ohmi, E. Hasegawa and M. Ohwada, J. Appl. Phys., 68 (1990) 1272, and T. Yasaka, K. Kanda, K. Sawara, S. Miyazaki and M. Hirose, Jpn. J. Appl. Phys., 30 (1991) 3567] and an extremely thin oxide film forming process through use of photoelectron spectroscopy, infrared absorption spectroscopy, thermal desorption spectroscopy and so forth. As regards the fluctuation of electrical characteristics by the interface level which becomes a discrete quantity with the ultraminiaturization of MOS FETs, the possibility of such fluctuation has been pointed out, besides an example of measurements of capture and emission of electrons by traps commonly referred to as electron noise has been reported [M. Schulz and A. Kramann. Physica Scripta. T35, (1991) 273]. There has not been made any report which demonstrates the clear existence such fluctuations of the electrical characteristics in ultrafine microstructures by introducing the interface level or traps purposely.
With respect to the SiO.sub.2 /Si interface system, the inventor of this application has tried to gather findings mainly on the atomic arrangement in the interface through use of an ultrahigh resolution electronic microscope and strain or distortion energy calculations based on interfacial structure models [H. Akatsu and I. Ohdomari, J. Non-Grist. Solid., 89 (1987) 239, H. Akatsu and I. Ohdomari, J. Phys., 62 (1987) 3751] and found that a high correlation does not always exist between the interface undulation on an atomic scale and the interface level or trap density. At present, he wonders if the interface level or trap density is not governed rather by factors which have not been subject to control, such as hydrogen (H) in the interface [H. Fukuda, M. Yasuda, S. Kaneko, T. Ueno and I. Ohdomar i, J. Appl. Phys., 72 (1992) 1906]. By the single ion implantation in the single ion implantation system and method according to the present invention, it is possible to introduce the interface level of a known density and clarify the behavior by the subsequent heat treatment in a hydrogen atmosphere.
(2) Impurity Control
As regards impurity control, there have been reported a number of examples which succeeded in improving the electrical and optical properties by forming a sheet-like impurity layer (by delta doping) during the crystal growth of a thin film (for example, Schubert [E. F. Schubert, J. Vac. Sci. Technol., A8 (1990) 2980]), but this is essentially control of the impurity distribution in the direction of growth; no report has been made on control of the number of impurity atoms in the ultrafine-microstructure.
The electrical conduction in a one-dimensional fine line or wire structure which is sufficiently small in size (on the order of .about.100 nm) in the direction perpendicular to the screen and has a length in the direction of travel to such an extent as not to cause scattering has been studied theoretically by many researchers including Landauer JR. Landauer, IBM J. Res. & Dev. 1, (1957) 223] and Buttiker [M. Buttiker, Phys. Lett., 57 (1986) 1761]. Recently, an interesting study of simulating variations in chemical and electrostatic potentials by impurities of such a system has been conducted by McLennan et al [M. J. McLennan, Y. Lee and D. Datte, Phys. Rev. B43 (1991) 13846].
There are many examples which actually fabricated quantum wires and confirmed the quantum size effect. In Japan, phenomena such as the quantization of conductance has been observed by Ikoma et al. who fabricated wires in a two-dimensional gas through utilization of defects induced by a focused ion beam (FIB) [T. Hiramoto, K. Hirakawa, Y. Iye and T. Ikoma, Appl. Phys. Lett., 51 (1987) 162] and Hirayama et al. who formed wires using the doping effect by FIB [Y. Hirayama, T. Saku and Y. Hirokoshi, Phys. Rev. B 39 (1989) 5535]. The quantum doping is a concept that has been proposed by the inventor of this application for the first time through implementation of the single ion implantation system and method according to the present invention.
(3) Device Control
Studies about the influence of irradiation of semiconductor materials and devices by a particle (ion) beam have been conducted mainly in the United States for the last 20 years or so, but these studies are intended for the use of devices in outerspace, nuclear facilities and under similar specific conditions. In 1979, however, it was reported by May et al. that alpha particles emitted from radioactive elements present in very small quantities in device packaging materials induced soft errors (device malfunction caused by the incidence of one high-energy ion) even in devices used under ordinary environmental conditions [T. C. May and M. H. Woods, IEEE Trans. Electron Devices ED-26 (1979) 2]. Furthermore, it was found that this problem would become more serious with a decrease in the size of devices; thereafter, semi conduct or manufactures began to earnestly pursue research about countermeasures against soft errors.
Conventionally, the evaluation of the ion irradiation effect on the device is conducted by randomly irradiating the device with ions emitted from a radioactive element or accelerator and statistically processing the frequency of malfunction of the device [N. Shiono, Y. Sakagawa, M. Sekiguchi, K. Sato, I. Sugai, T. Hattori and Y. Hirano, IEEE Trans. Nucl. Sci. NS-33 (1986) 1632]. This method possesses, however, the fatal defect of incapability of specifying the position of incidence of an ion to the specimen surface.
The direct evaluation of the ion irradiation effect on microminiatured integrated circuits requires a technique for conducting ion irradiation with resolution on the order of micromillimeter. In 1992 Horn, Doyle et al. reported a method for evaluating the ion irradiation effect on devices through utilization of high-energy ion beams focused to micron dimensions [K. M. Horn, B. L. Doyle and F. W. Sexton, IEEE Trans. Nucl. Sci. NS-39 (1992) 7]. This method permits specifying ion-irradiated positions but stops well short of the evaluation of a transient response because the irradiation of devices with a beam-focused high-intensity ion causes irradiation damage to semiconductor materials and permanent deterioration of the devices.
The inventor of this application and his colleagues evaluated the site dependence of the single ion irradiation effect on a present VLSI by use of a single ion irradiation effect T. Matsukawa, M. Koh, K. Hara, M. Goto and I. Ohdomari, Jpn. J. Appl. Phys. 31 (1992) L771]. This is the first attempt which irradiated a commercially available VLSI with ions each at a different position or site and evaluated the effect. They succeeded in measurements of the occurrence of soft errors in each site of 2 .mu.m square [K. Noritake, T. Matsukawa, M. Koh, K. Hara, M. Goto and I. Ohdomari, Jpn. J. Appl. Phys. 31 (1992) L771].
As described above, there have been developed so far new evaluation methods which induce soft errors by the irradiation of single ions to devices; but to achieve a direct understanding of transient response phenomena of devices and the production of indexes for the enhancement of their strength, it is necessary to examine the effects of irradiation with not only ions of helium (He) but also ions of various LETs (energies of ions lost per range) which are objects of the present invention. These objects can be attained by adding ion sources to a single ion microprobe already developed.
To obtain basic findings about transient phenomena of next generation ultrafine-microstructured devices of sizes under 0.1 .mu.m, the introduction of a system having an aiming accuracy on the order of 10 nm is indispensable. In this respect, an extremely high precision single ion implantation system could be implemented by introducing single ion extraction and aiming techniques, obtained in the development of the single ion microprobe, into a focused ion beam (FIB) system having a probe diameter under 50 nm.
Next, a description will be given of the prior art concerning the clarification of behaviors of point defects.
As regards research on crystal defects in semiconductors, especially silicon (Si), studies centering on electron spin resonance (ESR) and electrical measurement had been conducted very actively until the 1980's during which the ion implantation method invented in the late 1960's was established as a semiconductor manufacturing technique. However, since defects become more unstable as they become smaller, behaviors of interstitial defects and vacancy defects are still left unclarified. On the other hand, from the standpoint of computational physics, the smaller the defects, the more easily they can be handled; in 1984 Car et al. reported the results of theoretical calculations about the Bourgoin mechanism of interstitial atoms in silicon (Si) [R. Car, P. J. Kelly, A. Oshiyama and S. T. Pantelides, Phys. Rev. Lett., 52 (1984) 1814], after which many results of research have been reported, including a recent report on the influence of pressure on the diffusion of a dopant in silicon (Si) [O. Sugino and A. Oshiyama, Marerials Science Forum, 83-87 (1992) 469].
The inventor of this application has already developed an ion irradiation system utilizing an ion microprobe. In Japanese Pat. Appln. No. 84904/92 filed in his name, there are disclosed a single ion irradiation system and method which enable a single ion or a predetermined number of ions to be applied an intended target point or site with high accuracy. The conventional ion irradiation system and method will be outlined below. FIG. 4 is a schematic diagram showing the principles of the prior art ion irradiation system.
The illustrated ion irradiation system is made up of: an ion microprobe for generating an ion microbeam 40; a deflector 41 for deflecting the ion microbeam 40; a beam chopper 42 for deflecting the ion microbeam 40 by the deflector 41; a micro-slit 43 for extracting therethrough a predetermined number of ions from the deflected ion microbeam; a specimen holder mechanism for holding a specimen 44 to be irradiated with ions; a secondary electron detecting system for detecting secondary electrons emitted from the specimen surface by its irradiation with ions, the secondary electron detecting system including a scanning electron microscope for observing the surface of the specimen 44 in real time. By instantaneous reversal of the direction of deflection the ion microbeam 40 with respect to the micro-slit 43 by the beam chopper 42, the predetermined number of ions are extracted through the micro-slit 43 from the ion microbeam 40, and one or more ions thus extracted are applied to the specimen surface.
The method for the operation of the above-mentioned system comprises the steps of: (1) deflecting the ion microbeam 40 in one direction to intercept it by the micro-slit 43; (2) instantaneously reversing the direction of deflection of the ion microbeam 40; and (3) detecting secondary electrons 46 emitted from the specimen 44 in a target chamber 45 by a secondary electron multiplier 47 to make sure the extraction and irradiation of the single ion.
The conventional ion irradiation system and method feature the ion generating technique with which it is possible to extract one or desired number of ions with a high degree of controlability. This is implemented by the combined use of well-known beam focusing and aiming techniques; important constituents are chopping for deflecting the ion microbeam and the slit width to extract a single ion, the micro ion beam current and the beam diameter. The inventor of this application found the conditions for extracting single ions with good controlability by repeating experiments for properly selecting the above-noted constituents. This made possible to extract a desired number of ions as well as single ions with a high degree of controlability.
Until then no proposals had been made on the system and method for extracting single ions or desired number of ions with high controllability, but the inventor of this application and his colleagues experimentally found out and made sure the method. The technique for generating one or predetermined number of ions with good controllability is not a conventionally known technique nor can it be easily obtained by combining known techniques.
With the combined use of the ion microprobe based on the known beam focusing technique and the known aiming technique for directing an ion stream to a target site, the above-mentioned ion irradiation system and method were implemented which enable a single ion or desired number of ions to be extracted with good controllability and enable the extracted single ion or desired number of extracted ions to be directed to a target site with high precision.
With the conventional ion irradiation system and method, only one ion can be directed to each site of one integrated circuit with desired energy; hence, they can be applied to the evaluation of the radiation hardness of the site by artificially causing phenomena (latch-up a CMOS, a soft error of a DRAM, etc.) through irradiation with the ion.
Since one ion can be directed to an aimed position with high accuracy, it is possible to measure the radiation hardness of a semiconductor device at each site and hence pinpoint the malfunction-prine location.
It is also possible to apply a single high-energy ion to a desired site of a target which has a microstructure, such as an LSI or chamber. Furthermore, it is possible to artificially cause malfunction of an integrated circuit by high-energy ions which naturally occur at random and infrequent intervals in ordinary use environments in the outer space or on the earth.
As the result of further development of the techniques obtained with the single ion irradiation system and method, the present inventor has developed a single ion implantation system and method.